From fundamental thermodynamics considerations, high efficiency conversion from heat to work requires both a high temperature heat source and a low temperature heat sink. The vast majority of energy conversion processes at present use the ambient surrounding here on Earth as the heat sink. On the other hand, outer space, at a temperature of 3 K, provides a much colder heat sink. Moreover, Earth's atmosphere has a transparency window in the wavelength range from 8 to 13 μm that coincides with the peak of the blackbody spectrum of typical terrestrial temperatures around 300K, enabling the process of radiative cooling, i.e. radiative ejection of heat from Earth to outer space, and hence the direct radiative access to this colder heat sink. Exploitation of radiative cooling therefore has the potential to drastically improve a wide range of energy conversion and utilization processes on Earth.
The study of radiative cooling has a long history. It has been well known since ancient times, that a black radiator facing a clear night sky can reach sub-ambient temperature. More recently, daytime radiative cooling under direct sunlight was demonstrated, where one used a specially designed radiator that reflects most of the sunlight but radiates efficiently in the atmospheric transparency window. However, the demonstrated performance thus far has been rather limited. For nighttime cooling, in typical populous areas the demonstrated temperature reduction from ambient air is on the order of 15-20° C. A temperature reduction of up to 40° C. has been demonstrated only at high-altitude desert locations with extremely low humidity. For daytime cooling, the demonstrated temperature reduction is approximately 5° C. An important open question then is: what is the fundamental limit of temperature reduction that can be achieved in typical populous areas on Earth?
Radiative cooling technology has been foreseen by Department of Energy as a strong candidate to complement existing cooling technology, e.g. air conditioning. Due to its passive nature, radiative cooling technology does not consume electrical power, nor does it emit greenhouse gases. However, the performance achieved thus far is rather limited, which hinders the wide application of radiative cooling.
What is needed is a radiative cooler that demonstrates a temperature reduction that far exceeds what is known in the art.